Patients afflicted with movement disorders or other neurodegenerative impairment, whether by disease or trauma, may experience muscle control and movement problems, such as rigidity, bradykinesia (i.e., slow physical movement), rhythmic hyperkinesia (e.g., tremor), nonrhythmic hyperkinesia (e.g., tics) or akinesia (i.e., a loss of physical movement). Movement disorders may be found in patients with Parkinson's disease, multiple sclerosis, and cerebral palsy, among other conditions. Delivery of electrical stimulation and/or a fluid (e.g., a pharmaceutical drug) by a medical device to one or more sites in a patient, such as a brain, spinal cord, leg muscle or arm muscle, in a patient may help alleviate, and in some cases, eliminate symptoms associated with movement or other nervous disorders.
During a programming session, which may occur during implant of the medical device, during a trial session, or during a follow-up session after the medical device is implanted in the patient, a clinician may generate one or more therapy programs that provide efficacious therapy to the patient, where each therapy program may define values for a set of therapy parameters. A medical device may deliver therapy to a patient according to one or more stored therapy programs. In the case of electrical stimulation, the therapy parameters may define characteristics of the electrical stimulation waveform to be delivered. Where electrical stimulation is delivered in the form of electrical pulses, for example, the parameters may include an electrode combination, an amplitude, which may be a current or voltage amplitude, a pulse width, a pulse shape, and a pulse rate.
The sensing and monitoring of electrical signals from the patient's nervous system can be an important aspect of therapeutic and diagnostic procedures. However, such sensing and monitoring presents challenges. Neurological bioelectrical signals have relatively small magnitudes compared to those in other areas of the body, such as cardiac signals, for example. Accordingly, these signals are typically amplified for analysis. However, the application of electrical stimulation obscures these neurological bioelectrical signals during the application of the stimulation. For example, saturation of the amplifiers results, thereby rendering the sensed signal unreliable. As a result, bioelectrical signals are not typically measured during stimulation. Instead, sensing electrodes are often blocked during stimulation and re-enabled after stimulation in an attempt to observe the effect of the stimulation pulse. However, such blocking and re-enabling practice can result in missing useful information that occurs during the stimulation itself.
In addition, sensing signals from the patient's nervous system can be affected by properties of the electrode-tissue interface. For instance, a mismatch in the electrode-tissue interface impedance between a pair of sensing electrodes can lead to limited ability to sense desired electrical signals. Moreover, interface impedances between electrodes and nervous tissue can be unpredictable and therefore cannot be “matched” while placing electrodes in contact with the nerves. Thus, the sensing of electrical signals from a patient's nervous system can be complicated by properties of the electrode-tissue interface.